In accordance with recent developments in technology, various types of personal computers (hereinafter also referred to as "PCs"), such as desktop and notebook types, have been produced and are being sold on the market. Among these PCs are compact and light notebook PCs that are designed and produced while taking into consideration their suitability for use in mobile environments, i.e., their portability and their adequacy for outdoor use.
One of the features of a notebook PC is a "battery-operated type," i.e., it can be driven not only by a commercially available power source but also by an incorporated battery. This is because a notebook PC is so designed that it can be used in a place, such as outdoors or at a destination on a business trip, where a commercially available power source may not always be available for employment. The incorporated battery is generally a "battery pack," consisting of a package containing a plurality of cells. Rechargeable batteries, such as Li-Ion or NiMH, are used for the battery cells.
The capacity of a battery incorporated in a PC is limited, and the battery operating life is typically two to three hours. Therefore, various ideas have been proposed for extending the battery operating life. The technique employed for the so-called "power management" process is the best example of a method for extending battery operating life. The battery operating life can also be easily extended by increasing the number of cells in a battery pack, or by employing battery cells having a high energy density, such as Li-Ion battery cells.
Recently, a PC for which two battery packs can be provided, i.e., a "dual battery" PC, has been developed. A dual battery computer system is disclosed, for example, in Japanese Patent Application No. Hei 6-174387 (Japanese Unexamined Patent Publication No. Hei 8-54967), which was assigned to the present assignee. The "IBM ThinkPad 760E/760ED" ("ThinkPad" is a trademark of IBM Corp.), sold by IBM Japan, Ltd., is so structured that an auxiliary battery pack (also called a "second battery") can be replaced with another device, such as an FDD or a CD-ROM drive. A system having two battery packs will have a battery operating life twice that of a system having only one battery pack.
FIG. 3 is a schematic diagram illustrating the hardware block of a power supply system in a dual battery computer system. In FIG. 3, the power supply system includes not only a commercially available power source (an AC power source), but also two battery packs 10 and 20 that function as power sources. The voltage provided by the AC power source is converted into a DC voltage by an AC adaptor 40, and is input to a DC/DC converter 50 in the system. The output terminals of the battery packs 10 and 20 are parallelly connected to the DC/DC converter 50 with the output of the AC adaptor 40. The AC adaptor 40, the battery packs 10 and 20 can be removed from a system main body 100 and can be replaced with others. The DC/DC converter 50 regulates the AC adapter output voltage or the battery output voltage to an appropriate voltage level for driving a system load 60, and distributes the current to the respective loads in a system 60. The loads include a CPU 61, a main memory 62, a peripheral controller chip 63, such as a video controller, a liquid crystal display (LCD)/backlight unit 64, and a hard disk drive (HDD) 65.
To supply power to the system 60, generally, an AC power source that is regarded as having an endless capacity is employed. In other words, only when power is not supplied from the AC power source, i.e., when the AC adaptor 40 is not inserted into the AC power source, power is supplied by the battery packs 10 and 20, i.e., the discharging of the battery packs 10 and 20 is performed. Extra power supplied by the AC power source is employed to charge the battery packs 10 and 20. The charging/discharging of the battery packs 10 and 20 can be controlled by turning on/off analog switches 11 and 21 on the output lines of the battery packs 10 and 20. The analog switches 11 and 21 are operated by a charging/discharging controller 67 via control signals A and B. The charging/discharging controller 67 can detect the charging start/end time and the discharging start/end time for the battery packs 10 and 20 by, for example, measuring the output terminal voltages of the battery packs 10 and 20 and their ambient temperatures, or by adding the charge current or discharge current, which is generally well known in the art.
The charging cycle for a rechargeable battery generally includes a quick charge mode and a trickle charge mode. The quick charge mode is a mode for charging a battery, during a period immediately following the discharging of the battery, at a relatively high speed (i.e., with a large current) until it is almost fully charged by using a power source with constant current or constant voltage characteristics. The trickle charge is a mode for executing charging by using a microcurrent at a relatively low speed. One of the effects of trickle charging is that charging is performed with a microcurrent long enough to supplement a self-discharge during a period continuing until an almost fully charged battery is fully charged, so that, to the extent possible, deterioration of the battery can be prevented, even when the charging is performed for an extended period of time. The other effect of trickle charging is that an over-discharged battery can recover to a state wherein quick charging can be performed (hereinafter referred to as a "normal discharge state"). If quick charging is executed for a battery in an over-discharged state, deterioration of the battery's feature will occur. Recently, trickle charging is often performed in anticipation of the benefit conferred by the second effect mentioned above.
In a battery operated electronic apparatus, a charging/discharging controller (or a circuit having an equivalent function) monitors the charging/discharging feature of a battery pack and controls the charging/discharging operation.
FIG. 4 is a diagram illustrating the structure for monitoring the charging/discharging feature of a battery pack. The output voltage of the battery pack is extracted by an amplifier 81 parallelly connected to positive terminal T1. The ambient temperature of the battery pack is measured by a thermistor 82 provided in the periphery of the battery cell. One terminal of the thermistor 82 is connected to negative terminal T2 of the battery pack (i.e., it is grounded), and a voltage level at the other terminal indicates a value relative to the ambient temperature. The output current of the battery pack is obtained by a differential amplifier 83 and a resistor 84. The resistor 84, which is serially connected with the negative terminal of the battery pack, generates a voltage by a value equivalent to the output current, and the differential amplifier 83 amplifies the voltage and extracts the resultant voltage.
The output terminal voltage of the battery pack generally indicates a value relative to the remaining capacity of the battery. The temperature of the battery cell fluctuates in accordance with the charging/discharging process. The total of the electric charges that have flowed in or flowed out of the battery pack can be obtained by adding a charge current or a discharge current. Therefore, when the charging/discharging controller 67 monitors at least one of the outputs T3, T4 and T5 from the amplifier 81, the thermistor 82 and the differential amplifier 83, it can acquire the charge remaining on the battery pack and the charging state (well known).
The measurement electronic components 81, 82, 83 and 84 are, for example, provided in the battery pack, or are mounted on a printed circuit board near the battery pack in the electronic apparatus.
In the dual battery electronic apparatus shown in FIG. 3, when the charging/discharging controller 67 monitors the charging/discharging feature, i.e., one of the output terminal voltages, an output current value and an ambient temperature, for the respective battery packs 10 and 20, an adequate charging/discharging operation can be performed for each of the battery packs 10 and 20. For example, when the battery pack 10 is in an over-discharged state and the battery pack 20 is in a normal discharged state, trickle charging can be performed for the battery pack 10, and quick charging can be performed for the battery pack 20.
From the view point of power source management, monitoring and controlling the charging/discharging for each battery pack is very preferable. However, although it is possible for a high-end type apparatus, which has the electronic components for measuring an output terminal voltage, to obtain an output current value and an ambient temperature that are provided for each battery pack, this is difficult for a low-end type apparatus. Furthermore, the provisions for monitoring terminals T3, T4 and T5, in addition to electrode terminals T1 and T2, are difficult to include in the design of an apparatus, or are accompanied by an increase in the manufacturing costs.
If it can be premised that the battery packs 10 and 20 are almost equally charged and discharged, and the output terminal voltages of both battery packs 10 and 20 do not substantially differ, it is not necessary for the charging/discharging feature be monitored for the respective battery packs 10 and 20 described above, and monitoring the feature only at one position will be sufficient. For example, only when a voltage level is measured at a point P, where the output terminals of both battery packs are collected (see FIG. 3), is the charging/discharging feature for both battery packs 10 and 20 obtained. However, actually it is difficult to equally charge and discharge the battery packs 10 and 20, and the output terminal voltages of the two, i.e., their remaining capacities, tends to differ. Even when the voltage level at the point P in FIG. 3 indicates a normal discharged state, such an event as that the battery pack 10 is in an over-discharged state and the battery pack 20 is in a normal discharged state occurs relatively easily. Especially for battery packs that can be replaced, an unbalanced condition between the terminal voltages easily occurs. If quick charging is performed for the battery packs 10 and 20 in a condition where their discharged states are not uniform, deterioration of the performance of the over-discharged battery pack 10 will occur. When trickle charging is performed for both battery packs, the charging period will be increased.
To resolve the above shortcomings, it is one object of the present invention to provide a superior battery charger for a battery operated electronic apparatus, and to provide an electronic apparatus having a superior charging function.
It is another object of the present invention to provide a superior battery charger for executing trickle charging for a battery incorporated in a battery operated electronic apparatus, and to provide an electronic apparatus having a superior charging function.
It is an additional object of the present invention to provide a superior battery charger for appropriately executing trickle charging for an electronic apparatus incorporating two batteries or more, and to provide an electronic apparatus having a superior charging function.
It is a further object of the present invention to provide a superior battery charger for appropriately executing trickle charging for an electronic apparatus incorporating two batteries or more even when output terminal voltages of the incorporated batteries differ, and to provide an electronic apparatus having a superior charging function.